A metal oxide semiconductor such as titanium dioxide, zinc oxide and the like exhibits a property of absorbing light having an energy corresponding to its band gap. In recent years, such a metal oxide semiconductor has been paid attention to because of its high responsiveness due to holes and electrons generated by excitation caused by light irradiation so that there has been attempted to use such a semiconductor as a “photocatalyst” for its application to the remediation of the environment such as the purification of water, the prevention of pollution, sterilization, deodorization, the purification of atmosphere and the like. Of known photocatalysts, a commercial photocatalyst P25 (a product of Nippon Aerosil Co., Ltd.) or ST-01 (a product of Ishihara Sangyo Kaisha, Ltd.) that is titanium oxide has been widely used. However, the photodegradation activity is not sufficient in these photocatalysts. In particular, in order to utilize photocatalytic functions in an environment with the weak ultraviolet intensity such as indoors, a catalyst further excellent in the photodegradation activity has been demanded to be developed.
As a technique for improving the photodegradation activity, there have been widely known a technique for improving the shape of the photocatalyst represented by a particle diameter, a film thickness, a surface area, a pore diameter and the like, and a technique for adding a co-catalyst, a sensitizer and the like. Further, there has also been proposed a photocatalyst composited with a compound having other functions, while there has been known a photocatalyst composited with silica, aluminum oxide, zirconium dioxide, silica-alumina, zeolite and the like.
As a photocatalyst containing silicon oxide, there have been disclosed a photocatalyst particle with titanium dioxide coated with a porous silica film (refer to Patent Document 1) and a photonic semiconductor with a silica-based film loaded thereinto (refer to Patent Document 2). Of these two patents, in Patent Document 1, there have been disclosed that by compositing with a porous silica film, when a photocatalyst was added to the organic fiber, resin or the like by mixing or the like, an effect which hardly deteriorates an organic fiber, resin or the like could be provided and the photodegradation activity was equivalent to that of uncoated photocatalyst. In Patent Document 2, there have been disclosed that a silica-based film was formed by supplying an organohydrogen polysiloxane to a photocatalyst in gas phase and the fungicidal activity in light irradiation conditions was enhanced higher than the original activity of the photocatalyst even though the photocatalyst was coated.
As a production method for coating a photocatalyst with a silicon oxide film, there have been disclosed a method for coating the photocatalyst in a neutral organic medium by using alkoxysilane (refer to Patent Document 3), a method for coating it by partially neutralizing a silicate to weak basic region (refer to Patent Document 4), a method for coating it in an acidic or basic water-alcohol mixed medium by using alkoxysilane (refer to Patent Document 5) and a method for coating it by simultaneously adding a silicate and a mineral acid to a titania liquid dispersion, and maintaining the pH at 7 to 11 (refer to Patent Document 6).
There has been known a way of improving the photodegradation activity by making a composite of silicon oxide with a photocatalyst. There has been known an anatase titanium-silica composite subjected to a heating treatment of a co-precipitate generated by a hydrothermal reaction of an aqueous solution containing a sulfate of titanium and alkoxysilane.
There has been disclosed that the anatase titanium-silica composite exhibited the photodegradation activity over twice as high as a commercial photocatalyst ST-01 (refer to Patent Document 7 and Non-patent Document 1).
Patent Document 1: Japanese Patent Laid-open No. 1998-276706
Patent Document 2: Japanese Patent Laid-open No. 1987-260717
Patent Document 3: Japanese Patent Laid-open No. 2000-240719
Patent Document 4: U.S. Pat. No. 2,885,366
Patent Document 5: Japanese Patent Laid-open No. 1998-180115
Patent Document 6: WO 1993/022386
Patent Document 7: Japanese Patent Laid-open No. 2004-161592
Non-Patent Document 1: M. Hirano, et al., Chem. Mater: 2004, 16, 3725-3732